There has been proposed a lighting system having a configuration shown in FIG. 12 (for example, see JP2011-47779A; hereinafter referred to as “Document 1”). This lighting system includes: an object detection device 101 provided with a sensor 110 that detects the presence or absence of a detection target object in a detection area and outputs a sensor signal; and a lighting apparatus 102. The object detection device 101 controls a lighting state of the lighting apparatus 102.
The sensor 110 is a millimeter-wave sensor that transmits a millimeter wave to the detection area, receives a millimeter wave reflected by the detection target object moving in the detection area, and outputs a sensor signal having a Doppler frequency which is equivalent to a frequency difference between the transmitted millimeter wave and the received millimeter wave.
The object detection device 101 includes an amplifying circuit 111, a determining unit 112 and a lighting controller 113. The amplifying circuit 111 is configured to divide the sensor signal outputted from the sensor 110 into components according to a plurality of frequency bandwidths and to perform amplification for each of the plurality of frequency bandwidths. The determining unit 112 determines the presence or absence of the detection target object by comparing outputs of the amplifying circuit 111 with a predetermined threshold value(s). The lighting controller 113 controls a lighting state of the light apparatus 102 according to a determination result by the determining unit 112.
The object detection device 101 includes a frequency analyzer 114 and a noise rejection unit (a noise determining unit 115 and a switching circuit 116). The frequency analyzer 114 detects intensity for each frequency of the sensor signal outputted from the sensor 110. By using an analysis result by the frequency analyzer 114, the noise rejection unit reduces the influence of a noise having a particular frequency, produced constantly. A FFT (Fast Fourier Transform) analyzer is used as the frequency analyzer 114. The determining unit 112, the lighting controller 113 and the noise rejection unit are contained in a control block 117 mainly including a microcomputer. The amplifying circuit 111 constitutes a signal processor outputting the sensor signal component divided according to the predetermined frequency bandwidths. Document 1 also mentions that the signal processor may be constituted by an FFT analyzer or a digital filter.
The amplifying circuit 111 has a plurality of amplifiers 118, and an operational amplifier is used as each amplifier 118. In each amplifier 118, a frequency bandwidth of a signal component to be amplified can be set by an adjustment of various parameters of a circuit constituting the amplifier 118. That is, each amplifier 118 also functions as a band-pass filter permitting the passage of a signal component of a particular frequency bandwidth. Then, in the amplifying circuit 111, the plurality of amplifiers 118 are connected in parallel, and amplify the sensor signal components divided according to the plurality of frequency bandwidths. Then, the amplifiers 118 output divided and amplified signal components having the respective individual frequency bandwidth.
The determining unit 112 includes a comparator (abbreviated to “A/D” in FIG. 12) 119 for each amplifier 118. Each comparator 119 A/D-converts an output of a corresponding amplifier 118 into a digital value, and then compares the digital value with a predetermined threshold value, and thereby the determining unit 112 determines the presence or absence of the detection target object. In addition, individual threshold values are set for pass-bands (that is, for amplifiers 118) one each and each comparator 119 outputs a high-level signal when the output of the corresponding amplifier 118 is outside the scope of the threshold value. In this conventional example, a threshold value “Vth” of each pass-band in an initial state (before shipment) is a value represented by Vth=Vavg±Vppini, where the “Vppini” means a maximum of peak to peak “Vpp” in outputs “V” of each amplifier 118 detected within a given period of time, in a state where there is no electromagnetic wave reflection in an anechoic chamber, and the “Vavg” means the average value of said outputs “V”. The determining unit 112 includes an OR circuit 120, which obtains a logical add of comparison results from the comparators 119. If there is even one high-level signal, the OR circuit 120 outputs a sensing signal showing “sensed condition” which means the presence of the detection target object. In contrast, if all signals are low-level, the OR circuit 120 outputs a sensing signal showing “non-sensed condition” which means the absence of the detection target object. The sensing signal shows “1” in the sensed condition, and shows “0” in the non-sensed condition.
The noise rejection unit includes the noise determining unit 115 which determines, from the output of the frequency analyzer 114, the presence or absence of a noise having a particular frequency, produced constantly, and the switching circuit 116 which switches output states of amplifiers 118, to be output to the determining unit 112 according to a determination result of the noise determining unit 115.
The switching circuit 116 includes a plurality of switches 121, and each switch 121 is inserted between an amplifier 118 of the amplifying circuit 111 and a corresponding comparator 119 of the determining unit 112. All of the switches 121 are turned on in the initial state, and then on/off states of the switches 121 are individually controlled through an output of the noise determining unit 115, and thereby outputs from the amplifiers 118 to the determining unit 121 are individually turned on or off. That is, the switching circuit 116 turns off a switch 121 corresponding to an amplifier 118 having an arbitrary pass-band of pass-bands according to an output of the noise determining unit 115, and thereby an output of the amplifier 118 can be invalidated.
The noise determining unit 115 reads signal intensity (voltage intensity) of each frequency (frequency component) of the sensor signal which is outputted from the frequency analyzer 114, and stores respective signal intensity in a memory (not shown), and then determines, by using the stored data, the presence or absence of a noise having a particular frequency, produced constantly.
When the noise determining unit 115 determines that a noise having a particular frequency is produced constantly, the noise determining unit 115 controls the switching circuit 116 to turn off a switch 121 inserted between the determining unit 112 and an amplifier 118 having a pass-band containing the frequency of the noise. Thus, when the noise having the particular frequency is produced constantly, an output of the amplifying circuit 111 to the determining unit 112, corresponding to a frequency bandwidth containing the noise, is invalidated. In the conventional example, on/off state of each switch 121 is updated whenever the noise determining unit 115 determines that the surrounding environment is in “steady state”.
In a signal processing device for detecting movement of an object by processing sensor signals, Fast Fourier Transform (FFT) is used in general when removing a background signal such as unknown noise or undesired signal (a periodic signal that is present in the environment and is not derived from the movement of a detection target object) by adaptive signal processing in frequency domain.
In a case of converting, using FFT, a time-domain digital signal of which sampling period is “t” and of which sample number is “2N” into a frequency-domain signal so that the signal is useful for subsequent signal processing in view of aliasing characteristics after FFT processing, under a condition that the maximum frequency of a frequency range is “1/(2t) [Hz]”, the frequency resolution be “1/(2 Nt) [Hz]” and the number of valid samples be “N”. Further, a window function is applied before applying FFT. Therefore, an adaptive filter using FFT requires a memory having at least “2N” words, in order to apply only FFT other than adaptive processing. Further, because of requiring performing complex number calculation, this adaptive filter requires a comparatively large-scale hardware. Furthermore, the subsequent adaptive-processing and filtering processing requires performing complex number calculation, which causes larger load on the hardware.
Therefore, if the frequency-domain adaptive filter using FFT is applied to a signal processing device that processes a sensor signal, problems are raised that the load on the hardware and the cost thereof would increase. These problems of increasing load on the hardware and increasing cost has been one obstacle for the application of the filter to a commercial sensor device which typically requires a lowered cost.